Optical heterodyne and homodyne detection communication systems are well adapted to a long distance and large capacity optical communication system, because a FDM communication with a high receiving sensitivity and a high density of information is realized in the systems. In such a communication system, the compensation of a fluctuation of a signal light polarization is very important to increase a reliability in a practical use. This compensation suppresses the decrease of a receiving sensitivity which is caused at a receiving terminal by the change of a polarization of a signal light propagating through an optical fiber. This polarization change is resulted from a fluctuation of birefringence in the optical fiber which is induced by the change of an environmental temperature and an external pressure around the optical fiber. For the purpose of compensating a polarization fluctuation, an automatic polarization controlling method and a polarization diversity method are available.
In these two methods the automatic polarization controlling method has an advantage as compared to the polarization diversity method, in that the former is simpler in construction of a receiving system than the latter, and a receiving sensitivity of the former is higher than that of the latter Furthermore, the former has a possibility, in which the receiving system becomes much simpler in construction, because a collective polarization control of all received FDM signal lights can be carried out by use of a common polarization controller. These are described in the Japanese patent laid-open No. 62-171335 entitled "a method for propagating multi-wavelength lights through an optical fiber".
In general, where lights having different frequencies are propagated through an optical fiber, polarizations of the lights becomes different, even if the polarizations are the same to each other in a state prior to the propagation. For this reason, even if one of the polarizations is controlled in a selected channel to be constant among the signal lights at a receiving terminal, a receiving sensitivity is deteriorated in systems for receiving signal lights of the remaining channels, because the polarizations are not constant in the signal lights of other wavelengths.
This is induced in the conventional polarization methods as a disadvantage, and "Journal of Lightwave Technology, Vol. 6, No. 7, July 1988" explains this phenomenon as a polarization dispersion which is caused in a collective polarization control by a propagating delay time difference between principal modes of polarizations orthogonal to each other in a single mode optical fiber on pages 1185 to 1190 thereof in the report of "Polarization Dispersion and Principal States in a 147-km Undersea Lightwave Cable" by C. D. Poole et al.
Accordingly, it is an object of this invention to provide a method for controlling polarizations of signal lights in an optical FDM coherent communication system, in which a polarization dispersion is suppressed in a collective polarization control, thereby avoiding a deterioration of a receiving sensitivity.
According to this invention, a method for controlling polarizations of signal lights in an optical FDM coherent communication system, comprises steps of:
propagating frequency division multiplex signal lights through an optical fiber;
controlling polarizations of the frequency division multiplex signal lights collective-y; and
receiving the frequency division multiplex signal lights having collectively controlled polarizations separately by receiving systems including optical heterodyne or homodyne receivers;
wherein the polarization controlling step includes a step selected from steps of:
controlling relative polarization states of local oscillation lights in the receiving systems to coincide to each other;
receiving a control signal from one of the receiving systems which receives a signal light of a frequency to be allocated in the center of a frequency band of the signal lights or the vicinity thereof; and
dividing the frequency division multiplex signal lights into plural groups, polarizations being collectively controlled in each of the plural groups.
According to another feature of the invention, a method for controlling polarizations of signal lights in an optical FDM coherent communication system, comprises steps of:
propagating frequency division multiplex signal lights through an optical fiber at least one of the signal lights having a polarization orthogoral to those of the other signal lights;
controlling polarizations of the frequency division multiplex signal lights collectively;
receiving the frequency division multiplex signal lights having collectively controlled polarizations separately by receiving systems including optical heterodyne or homodyne receivers; and
controlling polarizations of local oscillation lights in the receiving systems, whereby a polarization of at least one of the local oscillation lights corresponding to the at least one of the signal lights is orthogonal to those of the other local oscillation lights.